The Reign of Terror came to an end, because the man who had started the Reign of Terror (Robespierre) was executed. The Reign of Terror also came to an end, because nuns had been executed because they did not want to give up their monastic vows and they were very calm during the execution, so this shifted the mood in Paris, probably making the general population realize that the mass executions were futile.
The Hundred Days Offensive Jul–Nov 1918 was the costliest
The Ludendorff offensive other wise known as the spring offensive was a German offensive on the Western Front intended to win the war before the American troops that had begun arriving in France could fully deploy.
I'm unsure of how far they advanced
Kaiserschlacht was the last German offensive and was a series of German attacks along the Western Front.
I think the answer is C :)